Annual variation of tissue biomass and carbon and nitrogen content in the freshwater bivalve Corbicula fluminea relative to downstream dispersal

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Abstract
Tissue dry weight biomass, ash-free dry weight, organic carbon content, nitrogen content, and organic carbon: nitrogen ratio (C:N) of individuals of Corbicula fluminea were monitored monthly over an annual reproductive cycle in a population living in an inlet from which a power station drew cooling water; the same variables were monitored for clams trapped on traveling screens in power station water intake embayments. These measurements were used to determine the relationship between tissue condition and downstream dispersal behavior. The ratio of dry tissue weight to dry shell weight showed marked seasonal variation: it was minimal during reproduction and maximal during overwintering, nonreproductive periods. Fractional ash content increased and organic content decreased during reproduction as shelled juveniles accumulated in the inner demibranchs. Seasonal variation in tissue organic C and N biomass corresponded directly to variation in dry tissue weight. Tissue C:N ratio was 6.0:1 in overwintering individuals and 4.9:1 during reproduction, indicating reduction of non-proteinaceous energy reserves. Reduction of the C:N ratio, dry tissue weight, and C and N biomass during reproduction suggests transfer of organic biomass to gamete production. Dry tissue weight, tissue organic C and N biomass, and C:N ratio were also reduced in individuals impinging on traveling screens, indicating poor nutritional status. Passive adult downstream dispersal onto traveling screens occurred only before periods during which clams incubated eggs and embryonic stages on the gills and released juveniles. Thus, downstream dispersal apparently allows relocation of individuals in poor nutritional condition to microhabitats more favorable for acquisition of energy for reproductive effort.